Carp will definitely work with Perl 5.6. You don't have to install it, but whoever is threatening to take away your customer's Linux support really needs to wise up - most Perl scripts will need some kind of module installed at some time or another and it's pretty childish to make blanket policies like that.
But this shouldn't be a problem because Carp is included in the core Perl distribution. So just use Carp; and you should be fine.
Are you sure it was a book? Are you sure it wasn't.....nothing?